Method and apparatus for determining location using a coarse position estimate

ABSTRACT

Corrections to a coarse position estimate of the pseudorange receiving device are made based upon knowledge of the amount of error present in inaccurate information (e.g., the old Almanac and/or Ephemeris) used to estimate the coarse position.

RELATED APPLICATIONS

[0001] This Application is a continuation of U.S. patent applicationSer. No. 09/773,207, filed on Jan. 30, 2001, Attorney Docket No. 000438.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The current invention relates to location of a device. Moreparticularly, the present invention relates to a method and apparatusfor determining the location of a device using a coarse estimate of theposition of the device and estimates of the location of transmittersused to determine that coarse estimate.

[0004] 2. Description of the Related Art

[0005] A common means by which to locate a device is to determine theamount of time required for signals transmitted from multiple sources atknown locations to reach a receiver within the device to be located. Onesystem that provides signals from a plurality of transmitters of knownlocation is the well-known Global Positioning Satellite (GPS) system.The position of the satellites within the GPS system can be identifiedby a number of different pieces of information, some more accurate thanothers. For example, a reasonably accurate determination of the locationof a GPS satellite can be made if the time of day at which a set ofinformation, known as the “Ephemeris”, is available. Transmissions fromeach satellite provide the time of day and the Ephemeris for theparticular transmitting satellite. The Ephemeris provides informationdefining the orbits of the particular satellites from which theEphemeris was received.

[0006] In addition, all of the satellites transmit another set ofinformation, referred to as “Almanac”. The Almanac includes lessaccurate information regarding the location of all of the satellites inthe “constellation”. Both the Almanac and the Ephemeris information isvalid for a limited amount of time. The Almanac information isconsidered to be accurate to approximately 3 kilometers forapproximately one week from the time the Almanac is transmitted. TheEphemeris provides information regarding the satellite orbits with anaccuracy of approximately 1 meter for a period of time equal toapproximately 2 hours. The errors in both the Almanac information andthe Ephemeris information grow as the data ages. Accordingly,information regarding the location of the satellites is less and lessaccurate as the Almanac and Ephemeris age, unless updated information isreceived at appropriate intervals in time.

[0007] Without accurate information regarding the location of thesatellites, the location that is determined based on the receipt ofsignals transmitted from the satellites will be inaccurate. Therefore,it is necessary to receive updates from the satellite or alternativelyfrom an alternative source. One such alternative source is a wirelesscommunication system base station that has a GPS receiver capable ofattaining the required information from the GPS satellites. However, forthe device to be located to attain the information at regular intervalsconsumes valuable resources, such as power required to receive theinformation, and bandwidth required to transmit the information from aremote source to the device. Accordingly, there is currently a need fora means by which accurate determinations of the position of a receivercan be made with a minimal expenditure of resources. This need isparticularly acute when using systems in which transmitters move overtime and the location of such transmitters is known accurately only uponreceiving updates from a source remote to the device performing thelocation calculation. The method and apparatus disclosed hereinsatisfies this need.

SUMMARY OF THE INVENTION

[0008] The presently disclosed method and apparatus allows the locationof a pseudorange receiving device (i.e., a coarse position estimate) tobe calculated using less accurate information. In one instance, the lessaccurate information is outdated Almanac or Ephemeris. As noted above,Almanac and Ephemeris information provide the location of positionlocation transmitters, such as GPS satellites. The use of inaccuratelocations for transmitters causes errors in the determination of thelocation of the pseudorange receiving device.

[0009] In accordance with the presently disclosed method and apparatus,corrections to the coarse position estimate of the pseudorange receivingdevice are made at a remote location at which more accurate informationis available. Alternatively, such corrections can be made at a remotetime when more accurate information is available. The disclosed methodand apparatus relies on knowledge of the errors present in the lessaccurate information (e.g., the old Almanac and/or Ephemeris) that wasused to calculate the coarse position estimate.

[0010] One advantage of the disclosed method and apparatus is that acoarse position location estimate and sufficient identification of theinformation used to calculate the coarse position estimate can beconveyed or stored in relatively short messages. In contrast, itrequires greater bandwidth to transmit to a location that has moreaccurate information, the measurements needed to calculate a moreaccurate estimate of the location. Similarly, it requires greaterstorage capacity to store the measurements until more accurateinformation is available. Accordingly, the present invention provides amethod and apparatus for saving either memory or bandwidth incalculating position estimates when accurate information is notavailable at the time or place where the measurements were taken.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0012]FIG. 1 is a simplified illustration of a system including apresently disclosed pseudorange receiving device.

[0013]FIG. 2 is a simplified block diagram of a pseudorange receivingdevice and a position identifying device in accordance with thedisclosed method and apparatus.

[0014]FIG. 3 is a flowchart illustrating the process by which theposition identifying device determines a more accurate position estimate(i.e., location of the pseudorange receiving device) based on a coarseposition estimate calculated within the pseudorange receiving device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0015]FIG. 1 is a simplified illustration of a system including apresently disclosed pseudorange receiving device 201. The pseudorangereceiving device 201 receives signals transmitted by a plurality oftransmitters 203 a-203 d (referenced collectively using reference number“203”) through an antenna 204. The pseudorange receiving device 201communicates with a position identifying device 207. The positionidentifying device 207 assists in determining the position of the pseudoreceiving device 201, as will be explained in more detail below.

[0016] In one embodiment of the present invention, the pseudorangereceiving device 201 is a cellular telephone capable of receivingsignals from the plurality of transmitters 203. However, it should beunderstood that the pseudorange receiving device 201 may be any devicecapable of determining the arrival times of received signals withrespect to a reference time. For example, the pseudorange receivingdevice 201 may be a computer terminal having a wireless modem, astand-alone GPS receiver, a receiver capable of receiving signals fromground based transmitters, or any other such receiver. It should also benoted that coarse position estimates made using inaccurate transmitterlocations together with actual ranges, as opposed to pseudo-ranges, maybe corrected using the disclosed method and apparatus.

[0017] The difference between a reference time and the time that thesignal is received is commonly known as the “pseudorange”. The referencetime may be any time, as long as the reference time is common to allpseudo-range measurements being made (or the pseudo-range measurementscan be adjusted to compensate for differences in the reference timesused). The term pseudorange is used to indicate that the exact amount oftime between transmission and reception cannot be determined, typicallydue to offsets in the clocks used in the transmitter and the receiver.

[0018] In addition to the actual position of each transmitter 203, FIG.1 illustrates an estimated position for each transmitter 205 a-205 d(referenced collectively using reference number “205”). In the instancein which the transmitters are satellites, such as GPS satellites, theposition of the satellites can be identified by a number of differentpieces of information, some more accurate than others. For example, areasonably accurate determination of the location of a GPS satellite canbe made if the time of day and information, known as the “Ephemeris” isavailable. However, the Almanac information is valid for a limitedamount of time. A less accurate determination can be made if accuratetime of day and current “Almanac” information is available. However,both Almanac and Ephemeris information are valid for a limited time.

[0019] The presently disclosed method and apparatus provides a means bywhich the location of a pseudorange receiving device 201 (i.e., a coarseposition estimate) can be calculated using less accurate informationregarding the location of satellites (such as an old Almanac or oldEphemeris). Since pseudoranges and the inaccurate locations of eachtransmitter are used to determine the coarse position estimate of thepseudorange receiving device 201, the errors in the locations of thetransmitters 203 will translate into errors in calculating the positionof the pseudorange receiving device 201.

[0020] Corrections to the coarse position estimate of the pseudorangereceiving device 201 are then made based upon knowledge of the amount oferror present in the less accurate information (e.g., the old Almanacand/or Ephemeris) that was used to calculate the coarse positionestimate.

[0021] In one embodiment of the presently disclosed method andapparatus, the corrections are made at the position identifying device207. The position identifying device 207 may be remotely located withrespect to the pseudorange receiving device 201. However, the positionidentifying device 207 may be co-located with the pseudorange receivingdevice 201 in some embodiments of the disclosed method and apparatus.

[0022]FIG. 2 is a simplified block diagram of a pseudorange receivingdevice 201 and a position identifying device 207 in accordance with thedisclosed method and apparatus. The pseudorange receiving device 201 maybe a component of a wireless communication system terminal, such as awireless telephone or a computer using a wireless modem. Alternatively,the pseudorange receiving device 201 may be a stand-alone positionlocation determining unit, such as a stand-alone GPS receiver. Theposition identifying device 207 may be a component (such as “PositionDetermining Equipment” commonly referred to as a “PDE”) of a wirelesscommunication system base station transceiver subsystem (BTS), basestation controller (BSC), or mobile switching controller (MSC) of awireless communication system. Alternatively, the position identifyingdevice 207 may be a component of, and co-located with, the wirelesscommunication system terminal or other device that includes thepseudorange receiving device 201. One example of a system in which theposition identifying device 207 and the pseudorange receiving device areco-located is a system in which accurate information about the locationof the transmitters 203 is not currently available when the measurementsare made. However, accurate information will become available at sometime in the future. Accordingly, a coarse position estimate iscalculated immediately upon making the pseudorange measurement (oractual range measurements). The coarse position estimate is then storeduntil more accurate information regarding the location of thetransmitters 203 becomes available.

[0023] The pseudorange receiving device 201 is coupled to an antenna204, and includes a receiver 303, a processing circuit 305, a processingmemory 307 and a communications port 309. The antenna 204 receivessignals from the transmitters 203 (shown in FIG. 1). The receivedsignals are coupled from the antenna 204 to the receiver 303. Thereceiver 303 includes all of the radio frequency circuitry (or othersuch receiving circuitry if the signals are not radio frequency signals)necessary to determine the intelligence in the received signals. Theintelligence is then coupled to the processing circuit 305. Theprocessing circuit 305 calculates positions. Such calculations mayinclude calculations intended to provide pseudoranges to the positionidentifying device 207. Alternatively, or in addition to thepseudoranges, position calculations may include coarse positionestimations that indicate the position of the pseudorange receivingdevice 201. Such coarse position estimates have a relatively large errordue to the large errors in the assumed positions of the transmitters203.

[0024] In accordance with one embodiment of the disclosed method andapparatus, the processing circuit 305 calculates position by executingprogram instructions stored within the processing memory 307. However,it will be understood by those skilled in the art that the processingcircuit 305 of the disclosed method and apparatus may include theprocessing memory required to store the program instructions, or thatthe processing circuit 305 may be a state machine or dedicated circuitrythat does not require program instructions to calculate the position.

[0025] The results of the position determination calculations arecoupled to the communication port 309 within the pseudorange receivingdevice 201. The communication port 309 is merely a communicationinterface that couples the results of the position determinationcalculations to a communication port 311 within the position identifyingdevice 207. Similarly, the communication port 311 is a communicationinterface between the communication port 309 and the positionidentifying device 207.

[0026] In addition to the communication port 311, the positionidentifying device 207 also includes a processing circuit 313 and aprocessing memory 315. However, it should be noted that the processingcircuit 305 may be capable of directly communicating with the positionidentifying device 207 without the need for a discrete communicationport. This may be the case in instances in which the positionidentifying device 207 and the pseudorange receiving device 201 areco-located. However, it should be understood that this may also be thecase in embodiments of the disclosed method and apparatus in which theposition identifying device 207 and the pseudorange receiving device 201are not co-located.

[0027] The communication port 311 within the position identifying device207 is coupled to the processing circuit 313. As is the case with thepseudorange receiving device 201, the processing circuit 313 of theposition identifying device 207 may be capable of communicating directlywith the pseudorange receiving device 201, making the communication port311 of the position identifying device 207 unnecessary.

[0028] The processing circuit 313 receives the results of the positiondetermination calculations performed within the pseudorange receivingdevice 201. In addition, processing circuit 313 within the positionidentifying device 207 receives a copy of the information that was usedby the pseudorange receiving device 201 to perform the positiondetermination calculations. It will be understood by those skilled inthe art that the information used by the pseudorange receiving device201 may be the Almanac (and the time at which the Almanac was valid),the Ephemeris (and the time at which the Ephemeris was valid), or anyother information that the processing circuit 305 within the pseudorangereceiving device 201 could use to estimate the location of thetransmitters 203 (shown in FIG. 1). This information may be provided bythe pseudorange receiving device 201 together with the results of theposition determination calculations. It should also be understood thatin some embodiments, it may not be necessary to make explicit whatinformation was used, since other indicators may be used to deduce whatinformation was used. For example, the time at which coarse positionestimates are transmitted to a position identifying device 207 may besufficient to allow the position identifying device 207 to correctlydeduce what Almanac or Ephemeris was used to calculate the coarseposition estimate. Furthermore, in some embodiments of the disclosedmethod and apparatus, the position identifying device 207 may beresponsible for transmitting to the pseudorange receiving device 201 theinformation used by the pseudorange receiving device 201 to calculatethe coarse position estimate. Therefore, there would be no need for thepseudorange receiving device to tell the position identifying device 207what information was used.

[0029] In one embodiment of the disclosed method and apparatus, theposition identifying device 207 receives this information prior toreceipt of the results of the position determination calculations.Furthermore, the information used by the processing circuit 305 may bereceived within the position identifying device 207 from a source otherthan the pseudorange receiving device 201 over a communication link thatis not shown for the sake of simplicity. For example, the informationmay be received directly by the position identifying device 207 from thetransmitters 203. Alternatively, the information may be received by theposition identifying device 207 from a source that is distinct from anyof the components shown in FIG. 1, such as a component (not shown) of awireless communication system base station.

[0030] In one embodiment of the disclosed method and apparatus, theposition identifying device 207 may have more than one set ofinformation (i.e., several versions of the Almanac, any one of which thepseudorange receiving device 201 may have used to perform the positiondetermination calculation). In this case, the pseudorange receivingdevice 201 may need to provide additional information to the positionidentifying device 207. Such additional information would indicate whatinformation, from among the information available to the positionidentifying device 207, was used by the pseudorange receiving device 201to perform the position determination calculation.

[0031]FIG. 3 is a flowchart illustrating the process by which theposition identifying device 207 determines a more accurate positionestimate (i.e., location of the pseudorange receiving device 201) basedon a coarse position estimate calculated within the pseudorangereceiving device.

[0032] As noted above, the position identifying device 207 firstreceives from the pseudorange receiving device 201, the results of theposition determination calculation (such as a coarse position estimate)(STEP 401). The position identifying device 207 also receivesinformation that indicates what data was used to perform the positiondetermination calculation (e.g., what version of the Almanac orEphemeris was used to determine a coarse position estimate) and the timeat which the pseudorange measurements were taken (STEP 403). Next, theposition identifying device 207 determines a more accurate estimate ofthe location of the transmitters 203 (STEP 405). This determination maybe made by using the Almanac and Ephemeris information that is mostaccurate for the time at which the pseudorange measurements were made bythe pseudorange receiving device 201. In one embodiment, the positionidentifying device 207 maintains a log of Almanac and Ephemeristransmitted by the satellites of the GPS constellation. Such a logallows the position identifying device 207 to use the most accurateAlmanac and Ephemeris to correct the position determination calculationreceived from the pseudorange receiving device 201 in the mannerdescribed below.

[0033] Once the position identifying device 207 has relatively accurateinformation regarding the position of the transmitters 203 andinformation regarding the position 205 (see FIG. 1) where thepseudorange receiving device 201 assumed the transmitters to be, theposition identifying device 207 can attempt to correct the positiondetermination calculation received from the pseudorange receiving device201.

[0034] Correction to the position determination calculation is made byfirst calculating an “equivalent pseudorange error”. An equivalentpseudorange error is calculated for each transmitter 203. FIG. 1 shows atransmitter at a point that represents the “more accurate” estimate ofthe location of the transmitter 203. FIG. 1 also shows transmitter atpoints 205 a-205 d (referenced collectively by the reference number“205”) that represent a less accurate estimate of the location used bythe pseudorange receiving device 201 to perform the positiondetermination calculations. It should be noted the transmitter 203 a andthe transmitter 205 a correspond to position location estimates for thesame satellites (as illustrated by the fact that the transmitter atlocation 205 a is shown in shadow). However, the location of thetransmitter is perceived to be different depending upon whether thelocation of the transmitter has been calculated using more or lesscurrent information (i.e., using Almanac and Ephemeris that is, or isnot, accurate for the time at which the pseudorange measurement wastaken).

[0035] The position identifying device 207 identifies a “transmitterposition error vector” 209 a-209 d (referenced collectively by thereference number “209”) associated with each transmitter 203, the vectorbeing formed between the two points 203, 205. The transmitter positionerror vector associated with each transmitter 203 is then projected ontoa unit vector in the direction of the line 211 a-211 d (referencedcollectively by the reference number “211”) between the transmitter 203and the pseudorange receiving device 201. This projection is representedin FIG. 1 by a dashed line 213 between the point 205 and a point on theline 211, the point on the line 211 being selected such that the line213 is perpendicular to the line 211.

[0036] It will be understood by those skilled in the art that theprojection has a magnitude represented by the two headed arrows 215a-215 d (referenced collectively as reference number “215”) shown inFIG. 1. This magnitude represents the value of the equivalentpseudorange error. This magnitude has either a positive or negativevalue depending upon the sign convention that is selected.

[0037] Due to the relatively large distance between the transmitters 203and the pseudorange receiving device 201 and the position identifyingdevice 207, the direction of the unit vector could alternatively bedefined by the line between the point 205 and the position identifyingdevice 207. The direction of the unit vector could also be defined bythe line between the point 205 and the pseudorange receiving device 201.Still further, the direction of the unit vector could be defined by theline between the point 203 and the position identifying device 207.Still further, the direction of the unit vector could be defined by theline between either the point 203 or 205 and a wireless communicationsystem base station with which the pseudorange receiving device is incommunication. All of these directions will be essentially identical,even though that does not appear to be the case in FIG. 1 because of thedistortion in the relative distances between the elements shown. Thatis, the distance between the transmitters and the devices 201, 207 ismuch greater than the distance between the points 203 and 205.Furthermore, the distance between the transmitters and either of thedevices 201, 207 is much greater than the distance between the devices201 and 207.

[0038] Once the value of the equivalent pseudorange error is calculatedfor each of the transmitters, that value is applied to a linearestimation algorithm, such as the well-known “Least Mean Squares”algorithm that is commonly used to determine the location of a GPSreceiver from pseudorange values to the GPS satellites (STEP 409).

[0039] The following is a more detailed description of the process usedto calculate the location of the pseudorange receiving device 201 basedupon: (1) knowledge of the inaccurate estimates of the location of thetransmitters, such as GPS satellites in one example, used to calculatean erroneous location for the pseudorange receiving device 201; (2) thetime at which the pseudorange measurements were taken; and (3) a moreaccurate knowledge of the location of the GPS satellites at the time thepseudorange measurements were taken.

[0040] It should be understood that there is an assumption that only theradial components of the transmitter location errors need to be takeninto account. The purpose of the presently disclosed method andapparatus is to attain an offset in the position of the pseudorangereceiving device 201 and add that offset to the position determinationcalculation provided to the position identifying device 207 by thepseudorange receiving device. For this example, the assumption is thatthe transmitters are GPS satellites. However, it will be clear to thoseskilled in the art that this process could be used with any mobiletransmitter for which location information is available which ages.$\begin{matrix}{{\overset{\_}{s}}_{i}^{a} = \lfloor {x_{Si}^{a}\quad y_{Si}^{a}\quad z_{Si}^{a}} \rfloor} \\{{\overset{\_}{s}}_{i}^{e} = \lfloor {x_{Si}^{e}\quad y_{Si}^{e}\quad z_{Si}^{e}} \rfloor}\end{matrix}$

[0041] is the almanac-derived location and b_(Si) ^(a) is the clockbias.

[0042] is the ephemeris-derived location and and b_(Si) ^(e) is clockbias for the i^(th) satellite at the time the pseudo-range measurementswere made.

[0043] {overscore (u)}^(a)=└x_(u) ^(a) y_(u) ^(a) z_(u) ^(a) b_(u) ^(a)┘is the location of the pseudorange receiving device 201 obtained by thepseudorange receiving device 201 when running a Least Mean Square (LMS)algorithm with pseudorange measurements made on the signals receivedfrom the satellites and satellite locations obtained from the almanac.

[0044] {overscore (u)}^(unit)=└x_(u) ^(unit) y_(u) ^(unit) z_(u)^(unit)┘ is an initial estimate of the user location. For this initialestimate, the location {overscore (u)}^(a) can be used. However, thoseskilled in the art will understand that this estimate can be derivedfrom other sources, such as the ranging measurements performed over aland communication network.

[0045] The measurement error due to the difference between the locationof the satellite i as determined based on the Almanac and the locationof the satellite i as determined based on the Ephemeris is given by:$ɛ_{i} = {{( {{\overset{\_}{s}}_{i}^{e} - {\overset{\_}{s}}_{i}^{a}} ) \cdot \frac{{\overset{\_}{s}}_{i}^{a} - {\overset{\_}{u}}^{init}}{{{\overset{\_}{s}}_{i}^{a} - {\overset{\_}{u}}^{init}}}} + {( {b_{S\quad 1}^{e} - b_{S\quad 1}^{a}} ) \cdot c}}$

[0046] where c is defined as being the speed of light.

[0047] These errors can be combined into the column vector {overscore(ε)}.

[0048] The geometry matrix is: $H = \begin{bmatrix}\frac{x_{u}^{init} - x_{S\quad 1}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{1}^{a}}} & \frac{y_{u}^{init} - y_{S\quad 1}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{1}^{a}}} & \frac{z_{u}^{init} - z_{S\quad 1}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{1}^{a}}} & {- 1} \\\frac{x_{u}^{init} - x_{S\quad 2}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{2}^{a}}} & \frac{y_{u}^{init} - y_{S\quad 2}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{2}^{a}}} & \frac{z_{u}^{init} - z_{S\quad 2}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{2}^{a}}} & {- 1} \\\vdots & \vdots & \vdots & \vdots \\\frac{x_{u}^{init} - x_{Sn}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{n}^{a}}} & \frac{y_{u}^{init} - y_{Sn}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{n}^{a}}} & \frac{z_{u}^{init} - z_{Sn}^{a}}{{{\overset{\_}{u}}^{init} - {\overset{\_}{s}}_{n}^{a}}} & {- 1}\end{bmatrix}$

[0049] Based on the linearity assumption, the optimal estimate of theposition error resulting from the satellite position errors is going tobe:

δ{overscore (û)} ^(T)=(H ^(T) ·R ⁻¹ H)⁻¹ H ^(T)·{overscore (ε)}

[0050] where R is the covariance matrix of the satellite measurements.The final estimate of the user position obtained using the ephemerisderived satellite position will therefore be:

{overscore (û)} ^(e) {overscore (u)} ^(a) +δ{overscore (û)}

[0051] Those skilled in the art will recognize that the validity of thiscalculation breaks down once the satellite position error becomes biggerthan a few kilometers.

[0052] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for calculating an accurate location ofa pseudoranges receiving device, comprising: a) calculating a coarselocation of the pseudorange receiving device using inaccurateinformation regarding the location of transmitters; and b) providingfrom the pseudorange receiving device to a position identifying devicethe calculated inaccurate location of the pseudorange receiving deviceand an indication of the content of the inaccurate information regardingthe location of the transmitters.
 2. The method of claim 1, wherein thetransmitters are GPS satellites and the inaccurate information isAlmanac information regarding the location of the GPS satellites.
 3. Themethod of claim 2, wherein the Almanac information is at least one weekold.
 4. The method of claim 1, wherein the transmitters are GPSsatellites and the inaccurate information is Ephemeris informationregarding the location of the GPS satellites.
 5. The method of claim 4,wherein the Ephemeris information is at least 2 hours old.
 6. The methodof claim 1, wherein the pseudorange receiving device is co-located withthe position identifying device.
 7. A method for calculating an accurateposition estimate based on a coarse position estimate, comprising: a)receiving the results of a coarse position determination calculation anda copy of information used to perform the coarse position determinationcalculation; and b) from the received information calculating correctionto the coarse position determination calculation. 8 The method of claim7, wherein the correction is used to calculate a more accurate positionestimate than is provided by the coarse position determination.
 9. Apseudorange receiving device comprising: a) a receiver; b) a processingcircuit coupled to the receiver, the processing circuit receiving fromthe receiver intelligence transmitted to the receiver from a pluralityof transmitters; and c) a communication port coupled to the processingcircuit, the communication port receiving from the processing circuitresults of position determination calculation and a copy of theinformation that was used by the processing circuit to perform theposition determination calculation.
 10. A position identifying devicecomprising: a) a communication port; and b) a processing circuit coupledto the communication port, the processing circuit receiving from thecommunication port the results of a position determination calculationand a copy of information used to perform the position determinationcalculation and from the received information calculating a moreaccurate position determination.